The present invention generally relates to superconducting devices. More specifically, the present invention relates to isolators based on nondegenerate three-wave mixing Josephson devices.
An isolator is a two-port device that transmits microwave or radio frequency power in one direction only. It is used to shield equipment on its input side, from the signals or noise coming from its output side. An isolator is a non-reciprocal device, with a non-symmetric scattering matrix and two ports. A port in this context is a plane where an external waveguide or transmission line (such as a microstrip line or a coaxial cable) connects to the device. An ideal isolator transmits all the power entering port 1 to port 2, while absorbing all the power entering port 2 destined for port 1. In other words, a signal applied to port 1 only comes out of port 2. However, a signal applied to port 2 is blocked from coming out of port 1. Within a phase-factor, the scattering matrix for an ideal frequency-preserving two-port isolator is
      [    S    ]    =            (                                    0                                0                                                1                                0                              )        .  
To achieve non-reciprocity, an isolator incorporates a non-reciprocal material. At microwave frequencies, this material is generally a ferrite that is biased by a static magnetic field. The ferrite is positioned within the isolator such that the microwave signal presents it with a rotating magnetic field, with the rotation axis aligned with the direction of the static bias field. The behavior of the ferrite depends on the sense of rotation with respect to the bias field, and hence is different for microwave signals travelling in opposite directions. Depending on the exact operating conditions, the signal travelling in one direction can either be phase-shifted, displaced from the ferrite, or absorbed.
Isolators are used in conjunction with superconducting circuits. New isolators are needed for superconducting applications without the constraints of current isolators that are based on magnetic materials and strong magnetic fields.